(1) Field of the Invention
This invention relates to a plate for lithography or offset printing and a process for the preparation thereof. More particularly, the invention relates to a plate for lithography or offset printing having a high resolving power and a high image density in combination and a process for the preparation thereof.
In the instant specification and claims, by the term "lithography" is meant a printing method in which a reverse image is formed on a plate and an ink image on the plate is directly transferred on a material to be printed, and by the term "offset printing" is meant a printing method in which a normal image is formed on a plate and an ink image on the plate is once transferred on a blanket roll and is then transferred on a material to be printed.
(2) Description of the Prior Art
Plates in which an oleophilic ink-supporting portion corresponding to an image to be printed and a hydrophilic ink-repelling portion corresponding to the background are formed on a suitable water-resistant substrate have heretofore been broadly used for lithography or offset printing.
These known plates for lithography or offset printing, however, are insufficient in the combination of the resolving power and image density. For example, plates formed by subjecting an electrophotrographic recording material having a photoconductive layer to charging and imagewise exposure to form an electrostatic latent image on the photoconductive layer, developing this latent image by a toner comprising a carbon black and a resin, fixing the developed image and then subjecting the photoconductive layer to an etching treatment, in which the toner-fixed portion acts as an ink-supporting portion and the background acts as an ink-repelling portion, have been broadly used for lithography or offset printing. These plates, however, have a relatively low ink-retaining property, and therefore, they have a defect that the image density of the resulting prints is ordinarily lower than that of prints obtained by relief printing or intaglio printing. Of course, the image density may be enhanced by increasing the amount of an ink applied to a plate for ligthography or offset printing, but if the amount of an ink is thus increased, the resolving power in prints is lowered because of the flowability of the ink, resulting in such defects as disarray of the image.